1. Field of the Invention
The present invention relates to cutting elements for use on earth boring drill bits and bits so equipped. In particular, the present invention relates to a cutting element which includes a support which interconnects an abrasive-impregnated cutting structure to the drill bit and mechanically reinforces the impregnated segment. More specifically, the cutting element of the present invention includes a tough and ductile support structure which may be internal or external to the impregnated segment.
2. Background of Related Art
Conventionally, earth boring drill bits with impregnated cutting structures, commonly termed "segments," have been employed to bore through very hard and abrasive formations, such as basalt, dolomite and hard sandstone. As depicted by FIG. 1, the impregnated segments 16 of such drill bits are typically secured to the boring end 14, which is typically termed the "face," of the bit body 12 of the drill bit 10 in a generally radial fashion. Impregnated segments may also be disposed concentrically over the face of the drill bit. As the drill bit gradually grinds through a very hard and abrasive formation, the outermost layer of the impregnated segments containing abrasive particles (such as small diamonds, diamond grit, or other superabrasive particles such as cubic boron nitride) wear and may fracture. Many conventional impregnated segments are designed to release, or "shed", such diamonds or grit in a controlled manner during use of the drill bit. As a layer of diamonds or grit is shed from the face, underlying diamonds are exposed as abrasive cuttings and the diamonds that have been shed from the drill bit wear away the exposed continuous phase of the segment in which the interior diamonds are dispersed, thereby "resharpening" the bit until the entire diamond-impregnated portion of the bit has been consumed. Thus, drill bits with diamond-impregnated segments typically maintain a substantially constant boring rate as long as diamonds remain exposed on such segments.
Conventional impregnated segments typically carry the super-abrasive particles in a continuous phase of a hard material, such as tungsten carbide, a tungsten alloy, a metal carbide, a refractory metal alloy, a ceramic, copper, a copper-based alloy, nickel, a nickel-based alloy, cobalt, a cobalt-based alloy, iron, an iron-based alloy, silver, or a silver-based alloy. Such materials are, however, typically relatively brittle and may fracture when subjected to the stresses of drilling. Accordingly, when subjected to the high stresses of drilling, and particularly impact stresses, the continuous phase of such impregnated segments may break, resulting in the premature failure thereof and potentially the premature failure of the bit upon which such segments are carried. Thus, drilling times and costs are increased by premature failure of conventional impregnated segments, as it is necessary to remove the drill string from the bore hole, replace the entire drill bit, and reintroduce the drill string into the bore hole.
U.S. Pat. No. 4,234,048 (the "'048 patent"), which issued to David S. Rowley on Nov. 18, 1980, discloses an exemplary drill bit that bears diamond-impregnated segments on the crown thereof. Typically, the impregnated segments of such drill bits are C-shaped or hemispherically shaped, somewhat flat, and arranged somewhat radially around the crown of the drill bit. Each impregnated segment typically extends from the inner cone of the drill bit, over the rose and up the bit face to the gage. The impregnated segments may be attached directly to the drill bit during fabrication, or partially disposed within a slot or channel formed into the crown and secured to the drill bit by brazing. When attached to the crown of a drill bit, conventional impregnated segments have a relatively low profile (i.e., shallow recesses between adjacent segments) relative to the bit face and a footprint that covers the majority of the drill bit surface from the nose to the gage. The low profile is typically required due to the relatively brittle materials from which the continuous phases of conventional impregnated segments are formed. Similarly, the generally semicircular shape of conventional impregnated segments and their somewhat radial arrangement around the crown of a bit body are required to prevent the breakage and premature wear of such impregnated segments due to the hard but relatively brittle continuous phase materials thereof. The large "footprint" of conventional impregnated segment-bearing drill bits is typically necessary to provide a sufficient amount of cutting material on the face of the bit. To some extent, the conventionally required semicircular shape of impregnated segments has also prohibited the use of alternative impregnated segment shapes, drill bit designs, and arrangements of impregnated segments on drill bits, which could otherwise optimize drilling rates and reduce the rate of bit wear and failure.
Because of the low profile or exposure and large surface area footprint of conventional impregnated segments, very little clearance exists between the face of the drill bit and the drilled formation during use of the drill bit upon which such segments are carried. Consequently, the build-up of formation fines, such as rock flour, on the impregnated segments may prevent contact of the impregnated segments with the interior surface of the bore hole, and may reduce the depth of cut of the drill bit. Moreover, due to the large surface area footprint and the low profile of impregnated segments on conventional drill bits, the hydraulics of such drill bits cannot be employed to remove formation fines therefrom or to cool the segments. Therefore, the rate of drilling and the amount of weight on bit that may be employed on the drill bit may be decreased, while the rate of wear is undesirably high, and failure of the drill bit may occur.
Thus, there is a need for an impregnated segment which will better resist breakage during drilling of very hard and abrasive formations, and which may be optimally designed and arranged upon a drill bit. There is also a need for impregnated segments which may be arranged on a drill bit to facilitate the use of drill bit hydraulics to remove formation fines from the impregnated surfaces of the drill bit and which facilitate the use of alternative drill bit designs.